Microchip fabrication involves the formation of integrated circuits (ICs) on a semiconducting substrate. A large number of semiconductor devices are typically constructed on a monolithic substrate of a single crystal silicon material. The semiconductor devices are formed by various processes such as doping and patterning the substrate and by depositing various conducting and insulating layers on the substrate.
The continued miniaturization of integrated circuits has brought about an increasing need to reduce resistivities in the source-drain-gate regions and of the contact metallurgy to these regions. Recently, much effort has been focused on the use of different conducting materials and metal silicides to form such contacts.
One conducting material that is used in such applications is titanium nitride (TiN). In the past, titanium nitride films have been used in semiconductor manufacture for local interconnects and as contacts to semiconductor devices. Recently, titanium nitride films have been used in advanced metallization technology for manufacturing ultra large scale integrated circuits (ULSI).
In these applications, titanium nitride may be used as a diffusion barrier against junction spiking for aluminum contacts to silicon. In addition, titanium nitride may be used as a glue layer between tungsten (W) and inter-metal dielectrics in a semiconductor structure. Titanium nitride may also be used to preserve the integrity of junctions in a semiconductor structure from worm hole effects during the chemical vapor deposition (CVD) of tungsten.
Improved processes have recently been developed for depositing conductive films such as titanium nitride, particularly for contact metallization. As an example, different methods of chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD) of titanium nitride have recently been developed.
In general, single phase titanium nitride films deposited onto a silicon substrate with a CVD process have a high contact resistance. For this reason, during metallization and contact formation using titanium nitride, a separate conductive layer of titanium is typically deposited on the silicon substrate. During the deposition process, the titanium layer is thermally reacted with silicon on the surface of the substrate to form a layer of TiSi.sub.x, such as titanium silicide (TiSi.sub.2). This process is sometimes referred to as silicidation. The layer of TiSi.sub.x can be used to form contacts having low contact resistance and good barrier properties.
This technology however, is not entirely suitable to the manufacture of ultra large scale integrated circuits (ULSI). Such high density circuits are formed with semiconductor devices having scaled down features. The contact junctions formed in the silicon substrate are thus relatively small and shallow in depth. Since the silicon in the junction region is consumed during the silicidation process in proportion to the thickness of the silicide that is formed, the shallow junction structure may be adversely affected during silicidation process. Specifically, it is difficult to make low resistance, reliable and thermally stable titanium silicide contacts to shallow junctions. Another problem associated with this technology is that reliable techniques for the deposition of highly conformal TiSi.sub.x films have not heretofore been developed.
In order to address these problems, various low pressure chemical vapor deposition (LPCVD) processes for depositing a layer of TiSi.sub.x on silicon have been proposed. As an example, LPCVD processes have been studied using a titanium precursor such as titanium tetrachloride (TiCl.sub.4) and a source of silicon such as silane (SiH.sub.4) to deposit TiSi.sub.x film.
A problem with such processes is that reaction temperatures in excess of 730.degree. C. are required. These temperatures are above the melting point of some of the materials used to form the semiconductor structure. Aluminum, for instance, melts at a temperature of about 600.degree. C. Another shortcoming of these prior art processes is that a seed layer of polysilicon may be required in order to deposit any TiSi.sub.x on a silicon substrate. Moreover, the films deposited in accordance with these methods have poor adhesion and rough surfaces.
Plasma enhanced chemical vapor deposition (PECVD) has also been utilized in the deposition of TiSi.sub.x films. Although a PECVD process can be performed at lower temperatures than the LPCVD processes previously described with PECVD, the bulk resistivity of the deposited films is relatively high (&gt;250 .mu..OMEGA.-cm). Furthermore, films deposited using PECVD are relatively rough.
As is apparent from the foregoing, there is a need in the semiconductor art for improved methods for depositing conductive films and particularly titanium silicide (TiSi.sub.x) films on silicon for contact metallization. Accordingly, it is an object of the present invention to provide an improved method for depositing conductive films such as TiSi.sub.x on a substrate utilizing pulsed-plasma enhanced chemical vapor deposition (PPECVD). It is a further object of the present invention to provide a pulsed plasma enhanced chemical vapor deposition (PPECVD) method in which high quality conductive films, and particularly TiSi.sub.x films can be conformally deposited on a substrate. It is yet another object of the present invention to provide a pulsed plasma enhanced chemical vapor deposition process for depositing TiSi.sub.x and other conductive films, on a substrate in which a deposited film is characterized by a smooth surface and a low bulk resistivity. Finally, it is an object of the present invention to provide a pulsed-plasma enhanced chemical vapor deposition (PPECVD) method for depositing TiSi.sub.x and other conductive films on a substrate that is suitable for large scale semiconductor manufacture and particularly semiconductor contact metallization for ULSI circuits.